The present Application claims foreign priority benefits of International Patent Application Number PCT/IB99/01478, filed Aug. 26, 1999, and French Patent Application Number FR98/10832, filed on Aug. 28, 1998, the contents of each application hereby being incorporated by reference.
The present invention relates to an implantable device for promoting fusion between two adjacent bony structures and a method of inserting the same. More particularly, the invention relates to an expandable fusion cage that may be inserted, in a reduced size configuration, into an intervertebral space and expanded after insertion to provide a desired size. While the device according to the present invention may have application in other areas of the body, the present invention is preferably utilized for vertebral interbody fusion.
There have been an extensive number of attempts to develop an exceptional intradiscal implant that could be used to replace a damaged disc and yet maintain the stability of the disc interspace between adjacent vertebra, at least until complete arthrodesis is achieved. These xe2x80x9cinterbody fusion devicesxe2x80x9d have taken many forms. For example, one of the more prevalent designs takes the form of a cylindrical implant. These types of implants are presented by the patents to Bagby, U.S. Pat. No. 4,501,269; Brantigan, U.S. Pat. No. 4,878,915; Ray, U.S. Pat. Nos. 4,961,740 and 5,055,104; and Michelson, U.S. Pat. No. 5,015,247. In the cylindrical implants, the exterior portion of the cylinder can be threaded to facilitate insertion of the interbody fusion device, as represented by the Ray, Brantigan and Michelson patents. In the alternative, some of the fusion implants are designed to be pounded into the intradiscal space. This type of device is represented by the patent to Brantigan.
Interbody fusion devices can be generally divided into two basic categories, namely solid implants and implants that are designed to permit bone in-growth. Solid implants are represented by U.S. Pat. Nos. 4,879,915; 4,743,256; 4,349,921; and 4,714,469. The remaining patents discussed above include some aspect that allows bone to grow across the implant. It has been found that the devices that promote natural bone in-growth achieve a more rapid and stable arthrodesis. The device depicted in the Michelson patent is representative of this type of hollow implant which is typically filled with a bone growth inducing substance to promote bone growth into and through the device. This implant includes a plurality of circular apertures which communicate with the hollow interior of the implant, thereby providing a path for tissue growth between the vertebral end plates and the bone growth material within the implant. In preparing the intradiscal space, the end plates are preferably reduced to bleeding bone to facilitate the tissue in-growth. During fusion, the metal structure provided by the Michelson implant helps maintain the patency and stability of the motion segment to be fused. In addition, once arthrodesis occurs, the implant itself serves as a sort of anchor for the solid bony mass.
One problem that is not addressed by the above prior devices concerns maintaining and restoring the normal anatomy of the fused spinal segment. Naturally, once the disc is removed, the normal lordotic or kyphotic curvature of the spine is eliminated. With the prior devices, the need to restore this curvature is neglected. For example, adjacent vertebral bodies may be reamed with a cylindrical reamer that fits the particularly implant. In some cases, the normal curvature is established prior to reaming and then the implant inserted. However, this over-reaming of the posterior portion is generally not well accepted because of the removal of load bearing bone of the vertebrae and because it is typically difficult to ream through the posterior portion of the lower lumbar segment where the lordosis is the greatest. In most cases using implants of this type, no effort is made to restore the lordotic curvature so that the cylindrical implant is likely to cause a kyphotic deformity as the vertebrae settles around the implant. This phenomena can often lead to revision surgeries because the spine becomes imbalanced.
In each of the above-listed patents, the transverse cross-section of the implant is substantially constant throughout its length and is typically in the form of a right circular cylinder. Other implants have been developed for interbody fusion that do not have a constant cross-section. For instance, the patent to McKenna, U.S. Pat. No. 4,714,469 shows a hemispherical implant with elongated protruberances that project into the vertebral end plate. Further, U.S. Pat. No. 5,669,909 to Zdeblick et al., shows a truncated conical implant adapted to be threadedly received in the intervertebral space. However, these devices require an opening at least as large as the largest segment of the device. The requirement for such a relatively large opening may limit the use of such devices, particularly where access to the spine is limited due to obstructing vessels and neurological structures.
Still further implants have been developed that provide the ability to adjust the size of the implant after insertion. U.S. Pat. Nos. 5,665,122 to Kambin, 5,554,191 to LaHille et al., and 5,653,763 to Errico et at., disclose implants which provide at least some degree of adjustability of the height of the implant to restore lordosis. However; these implants do not allow the device to be easily and securely inserted into a disc space and the internal expansion mechanism limits the ability to pack the interior with a large amount of bone in-growth material.
FR 2,753,368 discloses an interbody implant formed by a cage having four longitudinal branches that can be simultaneously radially expanded by a wedge means. Consequently the cage is laterally expanded in an undesirable manner, harmful to the patient.
WO 97/00054 teaches an implant in two separated members, connected by a complicated means with end portion shaped for maintaining an angle between these members.
U.S. Pat. No. 5,683,394 discloses an implant formed by an integral cage with an end cap.
U.S. Pat. No. 5,702,455 relates to an implant formed by two tubular members having holes and connected by a threaded tube.
In view of the limitations of the prior devices, there remains a need for an interbody fusion cage capable of stabilizing the spine in a manner comparable to interbody implant designs presently in use, and at the same time providing a mechanism for restoring normal lordosis of the spine. After expansion, the implant should have an internal cavity adapted to receive bone graft or bone substitute to encourage bone growth through the expanded implant.
In response to the needs still left unresolved by the prior devices, the present invention contemplates an expandable fusion cage adapted to be inserted between a pair of vertebral bodies to restore the normal angular relation between adjacent vertebrae. In particular, a device according to the present invention comprises a body having an outer bone engaging surface and an inner surface defining an internal chamber and a retaining mechanism. The body has a fixed portion and an expandable portion. The expandable portion is divided into at least a first branch and a second branch, interconnected at the fixed portion. The first branch is moveable with respect to the second branch in the expandable portion to expand the overall size of the fusion cage. An expansion member is sized to be at least partially received within the internal chamber and has an outer surface adapted to engage the retaining mechanism. Movement of the expansion member within the internal chamber urges the first branch to move with respect to the second branch thereby expanding the expandable portion. The expansion member is retained in position by engagement with the retaining mechanism of the body.
In another aspect of the invention, an expandable intersomatic cage has a tubular body having an outer bone engaging surface and an inner surface defining an interior chamber. The inner surface further defines an inclined surface and a retaining mechanism. The body defines a fixed portion and a moveable portion, the inclined surface is disposed adjacent the moveable portion. In this aspect of the invention, an expansion member is included having an outer surface configured to engage the inclined surface and the retaining portion. Movement of the expansion member against the inclined surface moves the moveable portion to expand the cage. The expansion member is retained in position by engagement with the retaining mechanism.
In another embodiment, the invention contemplates an insertion tool cooperable with an expandable fusion implant. The insertion tool is operable to insert the implant and move the expansion member to expand the cage. A cage according to the present invention is utilized with an insertion tool having an outer sleeve adapted to engage the body of the cage to transmit compressive force to the cage. The insertion tool further includes an inner shaft moveably disposed within the outer sleeve. The inner shaft is sized to be inserted into at least a portion of the interior chamber of the cage. The inner shaft has a distal end configured to removably engage the expansion member, whereby movement of the inner shaft with respect to the outer sleeve moves the expansion member in relation to the cage.
In another aspect of the invention, methods are provided for insertion of an expandable intersomatic device. In one aspect a device according to the present invention is inserted into the disc space. The moveable portion of the device is positioned adjacent a portion of the vertebra requiring additional spacing. The expansion member is then moved within the internal chamber and against the moveable portion to expand the cage to the desired height. The expansion member may be engaged with the retaining mechanism to limit movement of the expansion member.
One object of the present invention is to provide an implant that has a reduced size insertion configuration and is expandable from the insertion configuration to a larger configuration.
Another object of the present invention is to provide an expandable implant that has a substantially unobstructed interior chamber to receive bone growth promoting material.
Still another object of the present invention is to provide a fusion cage configured for easy insertion and expandable to a larger size to establish lordosis.
Yet a further object of the present invention is to provide an improved method for inserting a fusion cage and restoring lordosis.
Related objects and advantages of the present invention will be apparent from the following description.